This invention relates to the field of catalytic steam reforming of hydrocarbons to make hydrogen.
The hydrogen produced by the present invention may be used, for example, to operate a fuel cell. In a fuel cell, hydrogen is consumed during the process of producing electric power.
Steam reforming refers to the endothermic reaction whereby hydrogen is produced from methane, or from some other hydrocarbon. The steam reforming reaction, when the fuel is methane, is as follows:CH4+H2O→CO+3H2
For reforming a C8 hydrocarbon, the reaction is:C8H18+8H2O→8CO+17H2
If the fuel cell is to be used to power a vehicle, the fuel cell, and the steam reformer used to supply hydrogen to the fuel cell, must be compact. Also, steps must be taken to reduce or eliminate the carbon monoxide products, which will quickly poison the membrane in the fuel cell. The present invention provides a practical, compact catalytic steam reformer, which can be used on a vehicle, or in other environments where space is severely limited.
A problem encountered in steam reforming is the deposit of carbon on the reforming catalyst. Although elemental carbon does not explicitly appear in the equations shown above, it turns out that the steam reforming reaction does produce at least small amounts of carbon which, if allowed to build up, will eventually smother the catalyst and thereby render it useless. Carbon deposits are most likely to be produced when the fuel is a heavy hydrocarbon, such as a hydrocarbon having 12 or more carbon atoms per molecule. An example of such a heavy hydrocarbon is the fuel known as JP8.
Moreover, to the extent that less than all of the hydrocarbon fuel is converted in the steam reforming reaction, some of the unconverted fuel will enter the fuel cell downstream of the reactor. When unconverted fuel appears in the fuel cell, carbon deposits are likely to form, and the fuel cell may be rendered inoperative. The smaller the fraction of fuel that is converted in the steam reformer, the greater the likelihood that the performance of the fuel cell will be harmed. That is why it is essential, in a commercial steam reformer, to operate continuously at or near maximum efficiency.
The present invention therefore includes an embodiment which insures that the fuel will be fully converted, or nearly so, in the steam reforming reaction.
Another problem with steam reformers is their tendency to accumulate sulfur. This is especially problematic with reformers using JP-8 and diesel fuel. The sulfur deposits tend to accumulate mostly in the coolest areas of the reactor. The present invention includes means for reducing or eliminating the problem of sulfur deposits.